An investigation of the effect of sodium dodecyl sulfate on quasi-emulsifier-free emulsion polymerization for highly monodisperse polystyrene nanospheres

A quasi-emulsifier-free emulsion polymerization for the synthesis of highly monodisperse polystyrene (PS) microspheres with the diameter less than 100 nm has been developed, which is similar to the conventional emulsifier-free emulsion polymerization except for the addition of a small amount of sodium dodecyl sulfate (SDS) below its critical micelle concentration. To study the effect of SDS on the polymerization, we first explored the nucleation mechanism, and supposed that mixed micelles of SDS and surface active styrene oligomers may be formed and provide indispensable loci for the growth of PS primary particles. As the precursor particles grow, they become unstable and tend to adsorb the rest of SDS not participating in the formation of mixed micelles on their surfaces. Thereby, it reduces the coagulation tendency of the particles, and results in a significant increase of the particle number density which was found to be directly proportional to the square of the surfactant concentration, along with the increase of the polymerization rate and the decrease of the size of final PS particles. In addition, the presence of the surfactant will shorten the duration of the nucleation and, thus, lead to the highly uniform size of PS nanospheres.